1. Field of the Invention
The present invention relates to photovoltaic cells containing amorphous silicon. More particularly, the present invention relates to amorphous silicon photovoltaic cells wherein the n-layer has a wide optical bandgap.
2. Description of the Related Art
A conventional photovoltaic module includes a substrate upon which one or more photovoltaic cells are disposed. The photovoltaic cells include a front contact disposed on the substrate made of, for example, a metal oxide such as tin oxide, followed by a p-i-n junction and a back contact made of, for example, a metal such as aluminum. The p-i-n junction includes a layer of a semi-conductor material doped with a p-type dopant to form a p-layer, an undoped layer of a semiconductor material that forms an intrinsic or i-layer, and a layer of a semiconductor material doped with an n-type dopant to form an n-layer. Light incident on the substrate passes through the substrate, the front contact, and the p-i-n junction. The light is reflected by the rear contact back into the p-i-n junction.
In an amorphous silicon alloy p-i-n photovoltaic cell, the intrinsic layer is the photovoltaically active layer. That is, the intrinsic layer is the layer in which light is absorbed to create useful carriers. The phrase "useful carriers" means carriers that are collected to produce the photo-generated current in the photovoltaic cell. The photo-generated current is generated between the front and rear contacts of the photovoltaic cell.
Some of the incident light is absorbed by the doped layers (the p-layer and the n-layers) but the carriers generated in these layers have an extremely short carrier lifetime and recombine before they can be collected. Hence, absorption in the doped layers does not contribute to the photo-generated current in the photovoltaic cell and a minimization of absorption in doped layers enhances the short-circuit current of p-i-n photovoltaic cells. Light absorbed by the p-layer is in the portion of the visible spectrum having a short wavelength. As used herein, "short wavelength" means light having a wavelength on the order 390-450 nm. Absorption loss in the p-layer is a function of the bandgap of the p-layer. Thus, by adjusting the bandgap of the p-layer, the absorption loss in the p-layer can be minimized, for example, by using wide bandgap a-SiC:H p-layers.
An i-layer that comprises a-Si:H having a thickness on the order of 6,000 .ANG., for example, has an optical bandgap of 1.7 eV and an absorption co-efficient which is such that not all the incident light is absorbed in one pass through the thickness of an i-layer. The term "absorption coefficient" means the number of photons absorbed by a given material per unit length of that material. Light in the visible spectrum having a long wavelength, which is not absorbed in the first pass, is reflected back into the cell by the rear metal contact. As used herein, light of a "long wavelength" means light having a wavelength greater than 600 nm. This is true because the absorption co-efficient is a function of wavelength and the absorption co-efficient is high for light of wavelengths up to approximately 500 nm. The absorption co-efficient drops off for light of wavelengths greater than 500 nm to the extent that little of the long wavelength light is absorbed as it passes through the photovoltaic cell.
Thus, the long-wavelength light makes two passes through the n-layer. Depending on the thickness of the n-layer, usually on the order of about 500 .ANG., and its absorption co-efficient, which is normally higher than that of the i-layer, the light absorbed in the two passes through the n-layer does not contribute to the overall short circuit current of the photovoltaic cell.
In a p-i-n photovoltaic cell structure, the n-type doped layer has two functions: (1) it forms a rectifying junction with the i-layer, and (2) it forms ohmic contact with the rear contact. Both these functions require that the n-layer be highly conducting, that is, the layer must have a low electrical resistivity and a small activation energy. An n-layer with these characteristics facilitates the formation of a good rectifying junction between the i-layer and the n-layer and minimizes the contact resistance between the n-layer and the rear contact.
The present invention is intended to provide an amorphous silicon photovoltaic cell having an n-layer with a wider "effective" optical bandgap than conventional n-layers without detracting from the n-layer's conductivity characteristics. The term "effective" optical bandgap means the bandgap of the composite structure that comprises the n-layer of the present invention.
The present invention is also intended to provide a photovoltaic cell that includes an amorphous silicon n-layer of increased optical bandgap to minimize absorption of radiation as it propagates through the n-layer for the first time and after being reflected back through the n-layer. It is a purpose of the present invention to provide an n-layer of increased optical bandgap without decreasing the conductivity of the n-layer.
The present invention is further intended to provide a photovoltaic cell that includes amorphous silicon with an enhanced short circuit current.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the structure and method particularly pointed out in the appended claims.